1. Field of the Invention
The present invention relates generally to compensating for baseline wander in adaptive equalizer systems, and more specifically, to compensating for baseline wander in a high-speed, digitally-controlled adaptive equalizer system to facilitate computer communications in a local area network.
2. Background Art
Equalization restores a data waveform's frequency components which are lost when the waveform propagates through data transmission channels such as cables. Thus, equalization permits the received waveform to closely resemble the originally transmitted waveform. A typical application of an equalization scheme in the data communications art is to facilitate digital computer communication among workstations in a local area network (LAN).
The magnitude of frequency loss in a received waveform depends upon the length of the data transmission channel. Longer transmission channels cause losses across all frequencies but with greater losses in high frequency signals. Thus, the farther apart two workstations are in a LAN, the more likely the received data will be attenuated by frequency, shifted in phase (frequency dispersion), and attenuated with less Signal-to-Noise (S/N) due to crosstalk.
Adaptive equalizer systems determine and provide equalizations required for a received waveform to ultimately resemble the originally transmitted waveform. FIG. 1 shows a conventional adaptive equalizer system 100 in which workstation 102 transmits waveform 105 via transmission line 110 to workstation 115. Waveform 105 is typically the MLT3 three-level code signal.
MLT-3 is the IEEE standard line coding for 100BASE-TX. MLT-3 is classified as a tri-state, differential code. MLT-3 has three levels: +1 volt, 0 volt, and -1 volt, where the differential is measured relative to a common mode voltage, set by the transceiver's receive section. The common mode is a reference voltage level above ground, selected at the voltage level designated as "0 volt" on the MLT-3 waveform. The MLT-3 waveform transition occurs as follows: from 0V to +1V, from +1V to 0V, from 0V to -1V, and from -1V to 0V. For MLT-3, a transition occurs for each bit value of 1, and no transition occurs for each bit value of 0. MLT-3 was chosen by IEEE standards committee 802.3u for 100BASE-TX to meet FCC emissions requirements for frequencies above 30 MHz.
Transmission line 110 is typically unshielded twisted pair wiring. However, transmission line 110 may also include shielded twisted pairs, attachment unit interface (AUI) cables, copper distributed data interface (CDDI), coaxial transmission lines, or other types of wiring. Workstations 102 and 115 may also include other types of transmitters/receivers. Additional details on CDDI (FDDI) are discussed in Fibre Distributed Data Interface (FDDI)-Part: Token Ring Twisted Pair Physical Layer Medium Dependent (TP-PMD), American National Standard for Information Systems (Mar. 1, 1995) and in U.S. Pat. No. 5,305,350, both of which are fully incorporated herein by reference thereto as if repeated verbatim immediately hereinafter. The receiving end of transmission line 110 is connected through a data jack 120, such as an RJ45 jack, to the primary winding of a decoupling transformer 125 which decouples the received waveform 105'. The secondary winding of decoupling transformer 125 is connected to a transceiver chip 130 which includes an equalizer (gain stage) 135, a peak detector and comparator 140 and slicers 145 and 150.
A peak reference source 155 generates a "PEAK-REFERENCE" signal having a specific amplitude equal to the pre-propagation amplitude of waveform 105 at some frequency. Peak detector 140 compares the absolute amplitude value of received waveform 105' (at a specific frequency) with the amplitude value of the PEAK-REFERENCE signal and generates an "ERROR" signal based on the difference in amplitudes of both signals. The ERROR signal propagates, via feedback loop 142 with gain stage 144, to equalizer 135, which equalizes received waveform 105' to resemble originally-transmitted waveform 105.
Slicer 145 outputs via line 160 an output signal "SLICER1", while slicer 150 outputs via line 165 an output signal "SLICER2". The SLICER1 and SLICER2 signals slice equalized waveform 105' at predetermined voltage levels and are also driven into OR gate 167 which outputs a non-return-to-zero-inverted (NRZI) signal. (FIG. 2 shows the slicing levels of the SLICER1 and SLICER2 signals in received waveform 105'.)
In a conventional adaptive equalizer system 100 with a peak detector 140, peak reference source 155 generates the appropriate ERROR signal based on the following reference ratio: the received waveform 105' will have an amplitude value of 2.+-.5% volts for a transmission line 110 of zero-meter length.
Conventional adaptive equalizer system 100 has a problem with a phenomenon known as "baseline wander." Baseline wander occurs with a conventional 100BASE-TX adaptive equalizer system 100 using differential signal transmission (i.e., MLT-3 coding) over a twisted pair medium 110. For 100BASE-TX systems using MLT-3 coding, three voltage levels are used (i.e., +1V, 0V, and -1V) relative to a return voltage. This return voltage is typically set to ground and is referred to as the baseline or baseline reference. In practice, the baseline does not remain at ground, but, rather, wanders up and down. This wandering of the baseline is referred to as "baseline wander."
Baseline wander is a problem that occurs when a very long pulse propagates through an isolation transformer. Decoupling transformer 125 is a standard component on a card receiving waveform 105. Decoupling transformer 125 acts as a high-pass filter which typically prevents most frequencies less than four kilohertz from passing through to adaptive equalizer system 100. Decoupling transformer 125, acting as a high-pass filter, eliminates the DC component of the incoming waveform 105' and causes a long pulse to drift towards the common mode. This is known in the art as "DC droop."
When the secondary winding of decoupling transformer 125 decouples the received waveform 105' and sends the signal to the transceiver chip 130, the DC component of the original waveform 105 does not pass through. When an MLT-3 coded signal remains constant (i.e., there are no transitions), for periods longer than the cut-off frequency of decoupling transformer 125, the output of decoupling transformer 125 begins to decay to common mode. This phenomenon is caused by the inductive exponential decay of decoupling transformer 125. The wander of the baseline from ground affects all three MLT-3 signal levels equally and, for convenience, is tracked by the 0V signal. As explained above, the 0V signal is also designated as the common mode.
Because MLT-3 code has a transition for every 1 bit and no transition for every 0 bit, only constant 0 bits (not constant 1 bits) converted into MLT-3 code produce a baseline wander condition. Multiple baseline wander events result in an accumulation of offset which manifests itself either more at +1V or more at -1V, depending on the which direction the wander goes over time. Although certain data patterns can cause significant baseline wander, statistically random data has very little probability of baseline wander.
Before a data pattern enters an isolation transformer, the data undergoes 4B/5B encoding and scrambling. To spread the transmitted energy of the signal evenly, the FDDI TP-PMD (ANSI X3.263:1995) standard was adopted establishing 4B/5B coding of the data. The use of 4B/5B coding ensures that 1 bits and 0 bits are spread for each symbol. However, 4B/5B encoding tends to concentrate energy at the high end of the spectrum (31.25 MHz) because of the predominance of 1 bits. Although this situation is good for ameliorating baseline wander, the trade-off is that emissions are not evenly distributed. To further spread energy, scrambling of data is used as well. For Fast Ethernet, the ANSI X3T12-PMD standard was adopted which includes a method of scrambling the 4B/5B symbol codes.
After 4B/SB coding and scrambling, baseline wander for 100BASE-TX can occur because numerous runs of 0 bits can be generated by the scrambler. The scrambler generates numerous 0 bits when certain packets, known as "killer packets," enter the scrambler. The probability of a killer packet entering a scrambler is a small number out of all the possible data packet permutations (2.sup.12,000). Further, even if a killer packet enters the scrambler, a problem will arise only if the data pattern aligns with the scrambler seed. The probability of this happening is once out of 2,047 tries (1:2,047). Although statistically a rare occurrence in the real world, killer packets can be created in the laboratory to demonstrate the baseline wander problem.
What is needed is a system and method which can compensate for the problems associated with conventional adaptive equalizer systems experiencing a wandering baseline.